A convention strain sensor disclosed in Japanese Patent Laid-Open Publication No. 2000-180255 includes a glass layer and an over-glass layer.
The conventional strain sensor will be explained referring to the relevant drawings.
FIG. 7 is a side cross-sectional view of the conventional strain sensor, and FIG. 8 is a top view of the sensor. A metal plate 1 has a first fixing aperture 2 provided in one end region thereof, a second fixing aperture 3 provided in the other end region thereof, and a detecting aperture 4 provided in substantially the center region thereof. A glass layer 5 is provided on an upper surface of the metal plate 1 and four strain-sensitive resistors 6 provided on the upper surface of the glass layer 5. The strain-sensitive resistors 6 are electrically connected with a source electrode 7, a GND electrode 8, a first output electrode 9, a second output electrode 10, and a circuit pattern 11, thus constituting a bridge circuit. An overcoat glass layer 12 is provided on the strain-sensitive resistor 6, the circuit pattern 11, and the glass layer other than the source electrode 7, the GND electrode 8, the first output electrode 9, and the second output electrode 10.
A procedure of manufacturing the conventional strain sensor will be explained below.
First, glass paste is applied on the upper surface of the metal plate 1 by screen printing and is baked at about 850° C. to form the glass layer 5 on the upper surface of the metal plate 1.
Then, mixture paste including Ag and Pt is applied on an upper surface of the glass layer 5 by screen printing and is baked at about 850° C. to form the source electrode 7, the GND electrode 8, the first output electrode 9, the second output electrode 10, and the circuit pattern 11 on the upper surface of the glass layer 5.
Then, resister paste including Ru is provided between a portion of the glass layer 5 and a portion of the circuit pattern 11 and is baked at about 850° C. to form the strain-sensitive resistors 6 on upper surfaces of the glass layer 5 and the circuit pattern 11.
Finally, glass paste is applied by screen printing on upper surfaces of the strain-sensitive resistors 6, the circuit pattern 11, and the glass layer 5 other than the source electrode 7, the GND electrode 8, the first output electrode 9, and the second output electrode 10, and is baked at about 850° C. to form the overcoat glass layer 12 on the upper surfaces of the glass layer 5, the strain-sensitive resistor 6, and the circuit pattern 11.
An operation of the conventional strain sensor having the foregoing arrangement will be explained.
The metal plate 1 is fixed to a stationary base (not shown) with a bolt (not shown) which extends through the first fixing aperture 2 and the second fixing aperture 3 and is tightened with a nut (not shown). Then, a detecting member (not shown) is fixed at a detecting aperture 4. When the detecting member (not shown) is pressed from its upper by an external force F, the force deforms the metal plate 1, and the strain-sensitive resistors 6 on the metal plate 1 receives a stress or tension and have their resistances vary. The strain-sensitive resistors 6 and the circuit pattern 11 form the bridge circuit and are supplied with a voltage from the source electrode 7. Therefore, a difference between respective potentials at the first output electrode 9 and the second output electrode 10 is detected as a differential voltage, which has the external force F applied to the detecting member (not shown) be measured. In the conventional strain sensor, if pinholes are formed in the glass layer 5 and the overcoat glass layer 12, the pinholes allow water to reach the metal plate 1 and to disturb electrical insulation over the metal plate 1. For maintaining electrical insulation to the metal plate 1, plural glass layers 5 and overcoat glass layers 12 are provided.
However, the plural glass layers 5 and the overcoat glass layers 12 provided in the conventional strain sensor they may be hardly distinguished from each other, upper and lower, adjacent layers. The glass layers can be hardly identified, thus causing the manufacturing of the sensor not to be controlled easily.